1.7. pressu re david raju vundi pressure pressure is the amount of force applied to a known area. we...
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1.7.Pressure
David Raju Vundi
PressurePressure is the amount of force applied to a
known area.We can calculate it using the equation
Pressure = Force (N) / Area (m2)
We can reduce pressure by reducing the size of the
force or increasing the size of the area.
3
Pressure and DensityDensity = Mass/Volume
A property of the material.Pressure = Force/Area
Depends on the height of the fluid.Same in all directions.Units are:
Force/Area = N/m2.Pascals 1 Pa = 1 N/m2.Atmosphere 1 atm = 1.013 X 105 N/m2.
Pressure is due to the net force of the molecules in a fluid colliding with the walls.
A very large number of collisions happen each second.
Each collision exerts a tiny net force on the wall.
Aneroid Barometer
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Barometer– measures atmospheric pressure
vacuum
PatmPHg
barometerbarometer: device to measure air pressure
air pressure
vacuum
mercury (Hg)
Barometer
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401
Empty space(a vacuum)
Hg
Weight of themercury inthe column
Weight of theatmosphere(atmosphericpressure)
BarometerMercury filled
760 mm = 1 atm
Water filled10400 mm = 1 atm
The barometer measuresair pressure
Water column(34.0 ft. high
or 10.4 m)
Atmosphericpressure
Mercury column(30.0 in. high
or 76 cm)
BarometersMount Everest
Sea level On top of Mount EverestSea level
SAND
100g
I think the bigger
mass will sink
further
I think a bigger area will stop the
mass sinking
Pressure Experiment
Torricelli Barometer
The mercury in the tube pushes down with its weight.
The bottom of the tube is open to the atmosphere.
The air pushes on the open surface of the mercury.
On an average day, the pressure of the air equals the pressure exerted by a column of mercury 760 mm high.
Above 760 mm, there is a vacuum in the tube.
Weight of mercury
Barometer DetailWhy doesn’t the diameter of the
column of Hg make a difference?Recall that Pressure = force/area.The “force” is the weight of the
mercury, but the pressure that results is that weight divided by the area of the column. So … a bigger column weighs more but also has a proportionally bigger area, and the two factors cancel one another out.
The pressure caused by the column of mercury pressing down is independent of the diameter of the column.
Manometer
A manometer is comprised of a bulb containing a gas and a U-shaped tube.
The U-shaped tube is partially filled with mercury. The weight of the mercury puts pressure on the gas.
If the U-tube is OPEN there is also air pressure acting on the gas.
The gas molecules put pressure on the mercury.
PHg
Closed ManometersThere is a balance
between the weight of the mercury on the left (PHg) and the pressure of the gas on the right (Pgas).
The difference between the heights of the mercury on each side of the tube is a measure of the pressure of the gas.
Pgas = h
vacuum
PHg
Open ManometersWhen gas pressure is
greater than atmospheric pressure, the mercury is pushed toward the open end.
The balance is between the gas on the right, and the air plus mercury on the left.
Pair + PHg = Pgas
The weight of the mercury is measured as the height difference:
PHg = h
So Pgas = Pair + h
Pair
PHg
Open ManometersWhen gas pressure is less
than atmospheric pressure, the mercury is pushed toward the gas reservoir.
The balance is between the air on the left and the gas plus mercury on the right:
Pair = Pgas + PHg
The weight of the mercury is measured as the height difference:
PHg = hSo Pair = Pgas + h
Or Pgas = Pair- h
PHgPair
Sample Problems
PAIR = 765 mm
h = 27 mmh = 13 mm
PAIR = 753 mm
h = 20 mm
Pair = 790 mm
Find the pressure of the gas in each manometer.Pay attention to whether the manometer is open or closed!
1. 2. 3.
Sample Problem Answers1. Pgas+ h = Pair
Pgas= 790 mm - 20 mm = 770 mm Hg
2. Pgas = vacuum + 13 mm = 13 mm Hg
3. Pgas = Pair + h
Pgas = 765 + 27 = 792 mm Hg